I was a chart from this year and by a credited organization (I can't remember them right now though) and they said in 50 years we should hit 100 billion and be just shy of 11.
http://wotmania.com/messageboard2showmessage.asp?MessageID=416942 thats to my post about this issue
And below is the link to the article
http://www.cnn.com/2004/TECH/science/03/22/world.population.reut/index.html
The UN loves to overestimate usually and create disaster scenarios.
Anyway, I actually think we can have that many people without problems if we did a few things. The United States alone can support a population significantly higher than its current pop. So can places like Russia.